In the US, every ISP contract I've seen says you're not allowed to run a server from your home. And when they structure your connection to have 50mbps down and 0.5mbps up, that's the enforcement.
A lot of video-games have the host of a match actually work as a server. NAS as servers. In lots of inter-device applications, one of them works as a server.
Oh, right, and Xorg.